Machines utilize electrohydraulic steering systems because of their versatility over purely mechanical and hydraulic steering systems. In electrohydraulic steering systems, hydraulic power provides the muscle for the actual steering function via hydraulic components, while electrical components provide the command for the precision control of the hydraulic components. A redundant or secondary steering system may be implemented in concert with a primary steering system in order to take over control of the hydraulic components of the machine when necessary.
In one example, the redundant or secondary steering system may always be activated. Accordingly, there exists a need for a redundantly controlled electrohydraulic steering system that can turn the redundant or secondary steering system off.
A switching valve is disclosed in U.S. Pat. No. 6,173,728, entitled, “Switching Valve for a Hydraulic Power Steering System.” The switching valve of the '728 patent operates to direct hydraulic fluid to the steering control valve from either the primary pump or the auxiliary pump. More specifically, the '728 switching valve includes a spring that biases a spool of the switching valve to a neutral position. The spool of the switching valve is movable from the neutral position against the bias of the spring under the influence of a hydraulic fluid pressure differential.
However, the '728 patent does not utilize a redundant or secondary steering system. As such, the '728 switching valve only directs flow from the primary pump and the auxiliary pump straight to the steering control valve.